1. Field of the Invention
The present invention relates to a protection control system for a multilevel power conversion circuit of a flying capacitor type for AC motor driving and other applications.
2. Description of the Related Art
FIG. 4 shows a general type of power conversion circuit for converting a DC power to an AC power. A DC power supply DP delivers a voltage Ed between the positive electric potential terminal P and a negative electric potential terminal N. The DC power supply can be generally composed of an AC power supply system using a rectifier and a capacitor with a large capacitance, though those components of the AC power supply system are not shown in the figure.
The power conversion circuit comprises semiconductor switches Q1 through Q6 each composed of an IGBT and a diode, gate driving circuits GD1 through GD6 for driving the IGBTs, and a controller CNT. An example of a load on this power conversion circuit is an AC motor ACM. The semiconductor switches Q1 through Q6 are ON/OFF controlled by the gate driving circuits GD1 through GD6 according to the ON/OFF command of the controller CNT. The power conversion circuit of this construction can deliver an output voltage of the electric potential P or N of the DC power supply DP to the AC output terminal by the switching operation of the IGBTs. Thus, this converter is a two-level power conversion circuit.
FIG. 5 shows a four-level power conversion circuit of a flying capacitor type. This circuit comprises: six semiconductor switches, which are IGBTs, T1 through T6 connected in series between the positive electric potential terminal P and the negative electric potential terminal N of the DC power supply DP, a capacitor C1 connected between the collector of the IGBT T3 and the emitter of the IGBT T4, and a capacitor C2 connected between the collector of the IGBT T2 and the emitter of the IGBT T5. The capacitors C1 and C2 are called flying capacitors. The voltages across the capacitors C1 and C2 are controlled to be, in average, ⅓ Ed and ⅔ Ed, respectively, where Ed is the output voltage of the DC power supply DP. The connection point between the emitter of the IGBT T3 and the collector of the IGBT T4 is an AC output terminal in this circuit construction. This circuit is a four-level power conversion circuit that delivers the four levels of electric potentials given in the following.
Mode (1)Ed when T1, T2, and T3 are ONMode (2)Ed - 1/3 Ed when T1, T2, and T4 are ONMode (3)Ed - 2/3 Ed when T1, T5, and T4 are ONMode (4)0 + 2/3 Ed when T6, T2, and T3 are ONMode (5)0 + 1/3 Ed when T6, T5, and T3 are ONMode (6)0 when T6, T5, and T4 are ON
The modes (2) and (4) give an equal voltage, and the modes (3) and (5) give an equal voltage as well. But the capacitors C1 and C2 undergo either charging or discharging action. Thus, selectively controlling the output time allows the voltages across the capacitors C1 and C2 maintained, in average, at ⅓ Ed and ⅔ Ed, respectively.
Each IGBT is subjected to a voltage of Ed/3 in a steady state. Considering a surge voltage emerging in a transient phenomenon at a switching action, a substantially required withstand voltage for the IGBTs should be about ⅔ Ed, which is twice the steady state value.
FIG. 6 shows a five-level power conversion circuit of a flying capacitor type, which is an advanced form of the circuit of FIG. 5. Controlling the voltage across the capacitor C1 at Ed/4, the voltage across the capacitor C2 at Ed/2, and the voltage across the capacitor C3 at 3Ed/4, the power conversion circuit gives five levels of electric potential of Ed, 3Ed/4, Ed/2, Ed/4, and 0 (zero) at the AC output terminal.
FIG. 7 is a six-level power conversion circuit. Controlling the voltage across the capacitor C1 at Ed/5, the voltage across the capacitor C2 at 2Ed/5, the voltage across the capacitor C3 at 3Ed/5, and the voltage across the capacitor C4 at 4Ed/5, the power conversion circuit gives six levels of electric potential of Ed, 4Ed/5, 3Ed/5, 2Ed/5, Ed/5, and 0 (zero) at the AC output terminal.
FIG. 8 shows a power conversion circuit composed by blending a neutral point clamped type (an NPC type) conversion circuit with a flying capacitor type conversion circuit. This power conversion circuit comprises, in addition to the four-level power conversion circuit of a flying capacitor type shown in FIG. 5, a pair of series-connected IGBTs T7 and T8 in parallel to the capacitor C2, and a bidirectional switch composed of a pair of antiparallel-connected reverse blocking IGBTs T9 and T10, the bidirectional switch being connected between the point of series-connection of the IGBTs T7 and T8 and the point of series connection of a DC power supply DP1 and a DC power supply DP2. The power conversion circuit of FIG. 8 is a seven-level power conversion circuit. The examples of power conversion circuits mentioned above are disclosed in a Japanese Translation of PCT International Application No. 2009-525717, and Technical Report of The Institute of Electrical Engineers of Japan, No. 1,093 (in Japanese), FIGS. 2.2 and 2.3 in particular.
A short-circuited state usually occurs when a semiconductor switch composing a power conversion circuit breaks down for some reason. FIG. 9 shows short-circuit current running in a short-circuit fault in a two level inverter circuit. When a short-circuit fault occurs at the IGBT T2 in the circuit of FIG. 9 and then an ON command is given to the IGBT T1, a DC short-circuit current Ist flows in the path designated by the broken line in the figure. If this state continues for a certain period of time, the IGBT T1 also breaks down leading to a complete DC short-circuit state, increasing the damage of the power conversion system. In order to avoid such a situation, the gate driving circuit GD for each IGBT is usually provided with an arm short-circuit detecting circuit and a short-circuit protecting circuit that forcedly interrupts the IGBT upon detection of a short-circuit event.
FIG. 10 shows such a gate driving circuit. The gate driving circuit gives an ON/OFF signal for a gate-emitter voltage of the IGBT T0 through electrical insulation by a photo-coupler PC1. When an ON signal is given through the photo-coupler PC1, a transistor Qa turns ON and the positive side power supply Ep performs forward bias driving the gate-emitter voltage of the IGBT T0 through a resistance RG. Thus, the IGBT T0 is turned ON. When an OFF signal is given through the photo-coupler PC1, a transistor Qb turns ON and the negative side power supply En performs reverse bias driving the gate-emitter voltage of the IGBT T0 through a resistance RG. Thus, the IGBT T0 is turned OFF. A short-circuit protection circuit that forcedly interrupts the IGBT T0 in the event of overcurrent is composed of a diode Dc, a resistor R1, a capacitor Cd, a Zener diode ZD, a transistor Qc, and a diode Dd. If an overcurrent flows through the IGBT T0 in the period of ON signal, the collector-emitter voltage of the IGBT T0 rises causing a non-conducting state of the diode Dc. Consequently the transistor Qc turns ON and then the transistor Qb turns ON to interrupt the IGBT T0 forcedly. A photo-coupler PC2 is a short-circuit detecting circuit that feeds back information of overcurrent interruption event to a control circuit.
Although the example of FIG. 9 is a two-level circuit, a multi-level circuit as shown in FIG. 5 is also operated similarly. When the IGBT T3 (or T4) undergoes a short-circuit fault, if another normal IGBT T4 (or T3) turns ON, the capacitor C1 becomes short-circuited. Accordingly, the IGBT T4 (or T3) is forcedly turned OFF with the gate driving circuit thereof. When the IGBT T2 (or T5) undergoes a short-circuit fault, if another normal IGBT T5 (or T2) is in an ON state, the capacitor C1 and C2 becomes short-circuited. Accordingly, the IGBT T5 (or T2) is forcedly turned OFF with the gate driving circuit thereof. When the IGBT T1 (or T6) undergoes a short-circuit fault, if another normal IGBT T6 (or T1) is in an ON state, the capacitor C2 and the power supply DP becomes short-circuited. Accordingly, the IGBT T6 (or T1) is forcedly turned OFF with the gate driving circuit thereof. The circuits of FIG. 6 and FIG. 7 are similarly operated.
Now, the operation on the short circuit fault of the IGBT T3 in the circuit of FIG. 5 is more closely considered. Referring to FIGS. 11(a)-11(d), from the state (a) in which electric current from the DC power supply DP flows through the path of IGBT T1→T2→T3 to a load, the state transfers to the state (b) in which the IGBT T3 is short-circuited. When an ON signal is given to the IGBT T4 in this state (b), short circuit current Ist flows as indicated by the broken line. In that state, the gate driving circuit for the IGBT T4 detects the short-circuit event and forcedly interrupts the IGBT T4. At the same time, a short-circuit fault detection signal is transmitted through the photo-coupler PC2 in FIG. 10 to the control device, which delivers an interruption signal to every IGBT. As a result, the current running through the load flows in the state (c) of FIGS. 11(a)-11(d) through the path of: the diode of the IGBT T6→the diode of the IGBT T5→the capacitor C1→the IGBT T3. Since the IGBT T3 is in the short-circuited state in this time, the current flows through the capacitor C1 and the capacitor C1 continues to discharge until the voltage VC1 across the capacitor C1 decreases to zero volts at which the diode of the IGBT T4 turns to a conductive state. Thus, the current flows in the path indicated in the state (d) of FIGS. 11(a)-11(d). At this state, the voltage VC2 across the capacitor C2 is about 2Ed/3, and so the IGBT T2 is subjected to the voltage VT2=VC2≈2Ed/3.
FIGS. 12(a)-12(d) shows operation in the case of a fault of the IGBT T4, which is basically similar to the one shown in FIGS. 11(a)-11(d). Referring to FIGS. 12(a)-12(d), from the state (a) in which electric current from the load flows through the path of IGBT T4→T5→T6, the state transfers to the state (b) in which the IGBT T4 is short-circuited. When an ON signal is given to the IGBT T3 in this state (b), short circuit current Ist flows as indicated by the broken line. In that state, the gate driving circuit for the IGBT T3 detects the short-circuit event and forcedly interrupts the IGBT T3. At the same time, a short-circuit fault signal is transmitted through the photo-coupler PC2 in FIG. 10 to the control device, which delivers an interruption signal to every IGBT. As a result, the current running through the load flows in the state (c) of FIGS. 12(a)-12(d) through the path of: the IGBT T4→the capacitor C1→the diode of the IGBT T2→the diode of the IGBT T1. Since the IGBT T4 is in the short-circuited state in this time, the current flows through the capacitor C1 and the capacitor C1 continues to discharge until the voltage VC1 across the capacitor C1 decreases to zero volts at which the diode of the IGBT T3 turns to a conductive state. Thus, the current flows in the path indicated in the state (d) of FIGS. 12(a)-12(d). At this state, the voltage VC2 across the capacitor C2 is about 2Ed/3, and so the IGBT T5 is subjected to the voltage VT5=VC2≈2Ed/3.
In these cases, a withstand voltage of at least 2Ed/3 is required by the IGBT T2 in the case of fault of T3 and by the IGBT T5 in the case of fault of T4.
Actually, these IGBTs need a withstand voltage of about Ed that is the voltage of the DC power supply DP. In the normally operating state mentioned earlier, these IGBTs need only about 2Ed/3 that is twice the voltage steadily subjected to. However, the IGBTs are required to exhibit a withstand voltage higher than the value as described above. This leads to an enlarged size and an increased cost. Thus, there is a need in the art for an improved protection control system.